Echo cancellation in long distance telephonic communication transactions is known in the art. The need for echo cancellation, as is known in the art, arises from impedance mismatches associated with wireline telephone subscribers and an economic decision by telephone carriers to use two-wire connections between wireline subscribers and central telephone offices.
Two-wire connections, as is known, requires that a duplex telephone signal (transmit and receive) be mixed for exchange of signals between the central telephone office and the wireline subscriber. The mixing of transmit and received signals results in a portion of a received signal being re-transmitted as an outgoing signal from a receiving subscriber to a transmitting subscriber. While the re-transmitted signal may be perceived as a "hollow" sound to local communicators, the re-transmitted signal may represent a distracting echo in long distance communications.
The delay experienced by a subscriber between a transmission and an echo may be a determining factor in the acceptability and useability of the communication channel. Short delays experienced between local communicators (e.g. 1-20 milliseconds) typically doesn't represent an impediment to the efficient exchange of spoken words. Longer delays (e.g. 250-500 milliseconds), on the other hand, may result in syllables and, even, entire words being repeated as an echo and may render the communication channel unusable.
The advent of digital mobile communication systems has exacerbated the problem of time delays (and concurrent need for echo cancellation). Vocoder delays, convolutional coding algorithms, etc. typically introduce signal delays in mobile communication circuits, typically, in the area of 200 milliseconds.
The solution to the problem of echos, since the advent of the digital computer, has been through the construction of computer based echo cancellers. Echo cancellers have typically been based upon adaptive finite impulse filters (AFIRs) (see Adaptive Filter Theory, 2nd ed., by Simon Haykin, Prentice Hall, 1991). AFIRS provide for echo cancellation by generating a mathematical model of the echo characteristics of a communication system as a step in canceling the echo.
The mathematical model (adaptive echo canceller) as developed by Haykin (supra) includes an adaptive filter (filter vector) that operates on a reference sensor output (signal vector) to produce an estimate of the noise (echo), which is subtracted from a primary sensor output (signal, containing echo). The overall output of the adaptive echo canceller is then used to control adjustments made to tap values of the filter vector. The operation of the Haykin adaptive filter may be described in terms of three basic equations as follows:
1. Filter output ##EQU1##
2. Estimation error EQU e(n)=s(n)-y(n)
3. Tap-weight adaptation EQU f.sub.n+1 =f.sub.n +.mu.e(n)x.sub.n,
where the value, .mu., represents an adaptation constant.
The adaptation constant, .mu., (as taught by Haykin) is chosen to be as large a value as possible as a means of increasing a speed of filter convergence. Too large a value, on the other hand, leads to filter instability.
While the Haykin adaptive echo canceller works well within fixed transmission systems, difficulties are often experienced in changing environments, such as within a trunking system. Trunking systems, as is known in the art involve multiple and changing transmission parameters.
Where a trunking connection involves long time delays in signal transmission (caused by long distances in analog systems or vocoder processing times in a digital system) multiple echos may be present necessitating large filter vectors. Large filter vectors result in increased processing time and a decrease in convergence time. Where a filter is involved in a trunking operation the filter may be alternately switched in and out of circuits involving both long and short time delays and single or multiple echos.
Where short time delays (or single echos) are involved small filter vectors may be appropriate. Where long time delays, or multiple echos are experienced, a much larger filter vector may be needed. Because of the importance of echo cancellation, both to analog systems and to digital mobile communication systems, a need exists for an echo canceller with a convergence time less dependent upon trunking connection.